unrooting problem

so apparently my phone was rooted before I bought it (me not know what rooting was at the time). I found out that it has Superuser Permission installed but absolutely dose not work. So I'm trying to remove the app but seems it has proved more difficult than what I can do or seen on the internet to do. I have gone through what I think is the basic steps to remove apps. I tried deleting it from app manager to no avail. When I tried to use SBF, RSDlite it fails as soon as i push the start button. in the adb command line "adb remount" getting a message that says "remount failed: Operation not permitted". Typing in "adb shell rm \system\app/Superuser.apk"" getting a message that says "rm failed for \system\app\Superuser.apk, Read only file system" So my first question would be it seems since nothing I can see seems to work how do I take back administrative rights to be able to make this app read/write. second what do I do from there to remove the app.